1. Field of the Invention
The present invention relates to a via hole structure for interlayer connection formed in an insulating film and a process for the formation thereof. More particularly, in the present invention, fine via holes for interlayer interconnections are formed in a protective film, an interlayer insulating film, or other film of a multilayer interconnection board, a printed circuit for high-density mounting, a printed circuit board, a wiring board, an electronic component, etc. so as to have a shape such that in the subsequent step of depositing a metallic film for wiring, the metallic film can successfully fill up the via holes.
2. Description of the Related Art
In recent years, film of a polyimide resin has attracted attention as an organic insulating film. Although polyimide has heat resistance, an insulating property, and toughness, it does not inherently have a photosensitive property. For this reason, in order to form fine via holes in a polyimide film, it is necessary to use a process wherein etching is conducted through the use of a resist mask. This involves the following troublesome steps.
At the outset, a polyimide precursor to which no photosensitive property has been imparted is coated on a substrate such as a printed circuit board or a silicon wafer by spin coating, spray coating, roll coating, or the like. A photoresist is then coated thereon by the same coating method as that described above. The coatings are prebaked to evaporate the solvent contained in the polyimide precursor or photoresist to form a photosensitive plate. The photosensitive plate is then covered with a photomask having a via hole pattern formed thereon, and light such as an ultraviolet ray is applied from the top of the photomask to perform an exposure. This is then developed, thereby forming a resist pattern corresponding to the via hole pattern. In the development of the photoresist, since the photoresist is usually treated with a solvent, it is necessary to conduct post-baking after the treatment. Then, the resist pattern is used as a mask and subjected to plasma etching and wet etching to etch the polyimide film, thereby forming via holes in the polyimide film. Finally, the photoresist film is peeled off and heat-treated for imidation, thereby forming a polyimide film having via holes formed therein. At the time of the wet etching, since the polyimide film is subjected to isotropic etching, when via holes having a large aspect ratio (thickness of film/diameter of via hole) are formed, the upper side wall is unfavorably shaved and tapered, which hinders an increase in the density. This is because the taper angle of the wet etching is unconditionally determined and is as small as 40.degree..
A photosensitive polyimide which enables via holes to be formed without the aid of any photoresist has been developed for the purpose of solving the above problem and reducing the troublesome treatment step. The photosensitive polyimide imparts a photosensitive functional group to a molecule of the polyimide per se, causes an exposed region alone to be photoreacted to make the solubility of the exposed region lower than that of the unexposed region, and causes solely the exposed region, corresponding to the mask pattern, to be left following the development of the polyimide with a solvent. Further, rinsing is conducted through the use of ethyl alcohol or isopropyl alcohol, incapable of dissolving the exposed portion, thereby to completely remove (i.e., any residual dust of incompletely dissolved polyimide) dissolved dust. Finally, heat treatment is conducted to remove the photosensitive functional group having a poor heat resistance through depolymerization, thus causing a polyimide portion having a good heat resistance alone to be left for imidization. The method of introducing the photosensitive functional group varies depending upon the manufacturers of the material. Examples thereof include a covalent bond type and an ionic bond type.
FIG. 9 is a cross-sectional view of a multilayer interconnection board. In the drawing, 1 designates a substrate, 8 an insulating layer, 13 a wiring layer, and 7 a via hole for interlayer interconnection. The thickness of the coating of polyimide per each unit time of coating is as small as about 10 .mu.m, so that in order to impart a satisfactory insulating property to the insulating layer 8, it is necessary to conduct a plurality of recoatings. When a via hole 7 is formed in an insulating layer 8 comprising a plurality of successively coated polyimide film layers, it is useful for the via hole to be normally tapered as shown in FIG. 8(a) from the viewpoint of the formation of a metallic film in the subsequent step. In the case of the dry process, the metallic film for wiring is usually formed by vacuum deposition or sputtering. In this case, when the shape of the via hole is such that the side is vertical as shown in FIG. 8(b) or reversely tapered as shown in FIG. 8(c), it becomes difficult for the metallic film for the wiring to be deposited within the via hole, or there occurs a region within which the metallic film for wiring cannot be completely deposited.
In order to solve this problem, a proposal has been made of the following process for the formation of a via hole.
As shown in FIG. 6(a), a first polyimide film layer 2 is coated on a substrate 1 and subjected to selective exposure and development to form first via holes 4.
Then, as shown in FIG. 6(b), a second polyimide film layer 5 is coated thereon.
Thereafter, as shown in FIG. 6(c), a negative working glass mask 6, on which a pattern of via holes having a larger size than that of the first via holes 4 has been formed, is positioned so that the centers of the holes coincide with those of the first via holes 4, and the assembly is then exposed.
Then, after a development treatment is conducted as shown in FIG. 7(a), a curing treatment is conducted to form via holes 14 wherein the aperture of the via holes in the upper layer is larger than that of the aperture of the via holes in the lower layer as shown in FIG. 7(b).
Although the above-described via holes are widened upward, they are not of a tapered form. This often hinders a metallic film from completely filling up the via holes at the time of the deposition of a metallic film for wiring.
Accordingly, an object of the present invention is to eliminate the above-described drawback and to provide a via hole structure wherein via holes are formed in an insulating film of a multilayer interconnection board or the like so as to have a shape such that a metallic film can completely fill up the via holes when a metallic film for wiring is formed on the insulating film.